Question 2:
One of our customers have one storage wich is exactly the same like
the one wich is described on the Q1, but only used for samba storage
for storing media files (big files.)
I am sure, there is no torrent or similar, and i have told to the
customers on the beginning "don't write more files parallel, to
avoid fragmentation", but today the storage is >95% fragmented.

The customer sad, he only does file write one by one, and nothing more.
How can this be?

Depends on your samba configuration to how it does writes. If it is
conigured to do sync writes then it will have all sorts of
fragmentation problems. So without more details about the workload,
the kernel that is being used, the fragmentation occurring (xfs_bmap
is your friend) and the samba config, there's little that can be
suggested here.

In kernel, i have tried out the pdflush-patch wich let me set the number of
pdflushd-s to 8-64.
This was not a good idea i know that, but if i am right this have no effet
to the fragmentation because pdflush not works on the FS layer.